This invention relates to atomic force microscopes and, more particularly, in an atomic force microscope having a sample holder and a probe with a sensing tip, scanning means for creating relative movement between a sample in the holder and the sensing tip in orthogonal X,Y, and Z coordinate directions, sensing means for sensing the position of the sensing tip, and feedback means connected between the sensing means and the scanning means for creating a scan of the tip over a sample in the holder and for maintaining the tip in a pre-established relationship with respect to a scanned surface of the sample in the Z direction to obtain height information about the scanned surface of the sample, to the improvement to also allow material characteristics information to be obtained simultaneously about the scanned surface of the sample wherein the feedback means comprises, analog-to-digital conversion means for obtaining an analog signal from the sensing means and for converting the analog signal into a digital signal; digital computation means including program means for receiving the digital signal from the analog-to-digital conversion means and for calculating and outputting first digital vertical control signals which create desired relative movement between a sample in the holder and the sensing tip in the Z coordinate direction which maintain the tip in the pre-established relationship with respect to the scanned surface of the sample in the Z direction; and, first digital-to-analog conversion means for receiving the first digital vertical control signals from the digital computation means and for outputting analog control signals to the scanning means to affect movement thereof in the Z direction.
The atomic force microscope is a device which uses a very sharp tip and low forces to profile the surface of a sample down to atomic dimensions. Such a device, using a tunneling current sensor, is described in U.S. Pat. No. 4,724,318 by Binnig et al. An improved microscope which can operate with the tip in a fluid is described in a co-pending application by Hansma and Drake, Ser. No. 322,001, filed Mar. 13, 1989, now U.S. Pat. No. 4,935,634 and entitled ATOMIC FORCE MICROSCOPE WITH OPTIONAL REPLACEABLE FLUID CELL, which is licensed to the assignee of this application.
Basically, these devices have a tip on a flexible lever with the vertical position of the tip sensed by a detector. These detectors vary and have in the past been tunneling tips, optical beam deflection, or optical interferometers. Other sensors such as capacitive and inductive proximity detectors are possible. The principle of these microscopes is to scan the tip over the sample while keeping the force of the tip on the surface constant. This force is kept constant by moving either the sample or tip up and down to keep the deflection of the lever constant. In this way the topography of the sample can be obtained from this vertical motion and this data can be used to construct 3-dimensional images of the topography of the surface. In previous atomic force microscopes, an analog feedback circuit varied the height of the tip or sample using the deflection of the lever as an input.
Measuring the topography of a sample does not indicate the species of object on the surface. For instance, when measuring a biological sample it would be useful to measure the stiffness of the sample to separate, say, salt crystals from DNA or to separate the DNA from a hard surface such as glass that it may be lying on. Topography only measures shape, not stiffness.
Wherefore, it is the object of this invention to provide an atomic force microscope and method of operation which has the ability to map out both the local stiffness and the topography of a sample with nanometer resolution in order to better distinguish features in the data.
Other objects and benefits of the invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.